EP2246677B1 - Bolometric detector of electromagnetic radiation from the infrared to the terahertz spectral domain and detector array device comprising said detectors. - Google Patents

Abstract

The detector has a resistive load coupled to crossed bow-tie antennas (54, 68) to convert electromagnetic power into calorific power. A bolometric micro bridge (56) is suspended above a substrate (52) by support and thermal isolation arms (62). The micro bridge has a thermometric element coupled to the load so that temperature of the element is raised due to the effect of the calorific power. One of the antennas is located outside the micro bridge and capacitively coupled with the load. The other antenna is located in the micro bridge and resistively coupled with the load.

Description

FIELD OF THE INVENTION

The present invention relates to the field of bolometric antenna detectors, and more particularly to cross-butterfly antenna detectors for the detection of electromagnetic radiation in the range extending from the infrared, and in particular in the bands 3 - 5 microns and 8 - 14 μm, with terahertz.

• ■ le diagnostic médical, pour lequel la détection dans le térahertz autorise l'accès à des détails de structures anatomiques et les réactions chimiques qui s'y produisent, que ne fournissent ni les rayons X ni les ultra-sons ;
• ■ le domaine militaire et la sécurité aérienne, avec par exemple la réalisation de radars anti-furtivité ou de radars haute résolution permettant de faire de la discrimination ;
• ■ l'étude et la détection de la pollution atmosphérique, l'observation en ondes submillimétriques fournissant en effet des informations importantes sur la chimie atmosphérique et permettant ainsi un suivi inégalé des polluants atmosphériques comme par exemple le trioxyde d'azote qui est difficilement détectable par les techniques classiques car présentant de fortes raies d'absorption dans l'infrarouge lointain ;
• ■ l'identification d'espèces chimiques, de nombreux composés chimiques complexes ayant une signature dans la gamme terahertz suffisamment univoque pour permettre leur détection de manière certaine, comme par exemple certains explosifs et produits toxiques, certains composés issus de la maturation des fruits ou encore certains composés issus de la combustion industrielle ;
• ■ l'analyse de phénomènes moléculaires ou atomiques, la spectroscopie térahertz permettant d'obtenir de nouvelles informations sur des mécanismes comme la photoexcitation, la photodissociation et la solvatation. Il en va de même pour l'analyse d'interactions moléculaires (états vibratoires des molécules ou des liaisons hydrogènes par exemple), de systèmes à phase condensée, de processus dynamiques dans des grosses molécules telles que les peptides et les protéines ou encore l'observation de l'orientation des polymères avec une technique basée sur le rayonnement térahertz ;
• ■ l'étude des propriétés des matériaux, comme les semi-conducteurs, pour déterminer de manière non destructive par exemple leur mobilité, la dynamique de porteurs ultra-rapides et les interactions porteurs-phonons, les supraconducteurs, les polymères, les céramiques, les matériaux organiques et les matériaux poreux. De plus, dans la gamme térahertz, des matériaux tels que les plastiques, le papier et les textiles sont transparents, les métaux sont de parfaits réflecteurs et l'eau possède un grand pouvoir absorbant. Ainsi, la détection dans cette gamme est particulièrement adaptée à l'inspection de produits emballés ou au contrôle de procédés de fabrication in situ et en temps réel par exemple ; et
• ■ la télécommunication large bande, la course à des débits d'information toujours plus élevés, autant sur terre qu'entres satellites, poussant les industriels à développer des systèmes fonctionnant à des fréquences qui atteignent aujourd'hui plusieurs centaines de gigahertz et demain plusieurs térahertz.

The detection in the infrared range have many applications, widely known to date. With regard to the terahertz, that is to say in the frequency range between 100 gigahertz and 10 terahertz, the applications envisaged in a nonlimiting manner concern.

• ■ the medical diagnosis, for which the detection in the terahertz allows the access to details of anatomical structures and the chemical reactions which take place there, that do not provide the X-rays nor the ultrasounds;
• ■ the military and aviation safety, for example the creation of anti-stealth radars or high-resolution radar to discriminate;
• ■ the study and detection of atmospheric pollution, submillimetric wave observation providing important information on atmospheric chemistry and thus allowing unparalleled monitoring of atmospheric pollutants such as nitrogen trioxide which is difficult to detect by classical techniques because they have strong absorption lines in the far infrared;
• ■ the identification of chemical species, many complex chemical compounds with a signature in the range terahertz sufficiently unambiguous to allow their detection in a certain way, such as certain explosives and toxic products, certain compounds from the fruit ripening or else certain compounds resulting from industrial combustion;
• ■ the analysis of molecular or atomic phenomena, terahertz spectroscopy to obtain new information on mechanisms such as photoexcitation, photodissociation and solvation. The same goes for the analysis of molecular interactions (vibratory states of molecules or hydrogen bonds, for example), condensed-phase systems, dynamic processes in large molecules such as peptides and proteins, and the like. observation of the orientation of the polymers with a technique based on terahertz radiation;
• ■ the study of the properties of materials, such as semiconductors, to non-destructively determine, for example, their mobility, the dynamics of high-speed carriers and carrier-phonon interactions, superconductors, polymers, ceramics, organic materials and porous materials. Moreover, in the terahertz range, materials such as plastics, paper and textiles are transparent, metals are perfect reflectors and the water has a great absorbency. Thus, the detection in this range is particularly suitable for the inspection of packaged products or the control of manufacturing processes in situ and in real time, for example; and
• ■ broadband telecommunication, the race for ever higher information rates, both on land and between satellites, pushing industrialists to develop systems operating at frequencies that today reach several hundred gigahertz and tomorrow several terahertz .

STATE OF THE ART

Usually, a resistive bolometric detector measures the power of incident radiation in the infrared range and comprises for this purpose an absorbing resistive bolometric element which converts the luminous flux into a heat flow, which produces a temperature rise of said element with respect to a reference temperature. This increase in temperature then induces a variation of the electrical resistance of the absorbing element, causing voltage or current variations across it. These electrical variations constitute the signal delivered by the sensor.

However, the temperature of the absorbent element is usually largely dependent on the environment thereof, and in particular that of the substrate which comprises the electronic reading circuit. In order to insensitize the absorbent element as much as possible from its environment, and thus to increase the sensitivity of the detector, the absorbent element is generally thermally isolated from the substrate.

The figure 1 is a schematic perspective view of an elementary resistive bolometric detector 10 of the state of the art illustrating this principle of thermal insulation. This elementary detector, here in the form of a suspended membrane, is conventionally part of a matrix of elementary detectors with one or two dimensions.

The detector 10 comprises a thin membrane 12 absorbing the incident radiation, suspended above a support substrate 14 by means of two conductive anchoring nails 16 to which it is fixed by two heat-insulating arms 18. The membrane 12 usually comprises a layer of electrical insulator, such as for example SiO ₂ , SiO, SiN, ZnS or others, which provides the mechanical rigidity of the membrane 12, as well as a metal electrical interconnection layer deposited on the insulation layer.

A thin layer 20 of resistive material thermometric is also deposited in the center of the membrane 12 of the metal interconnect layer including a layer of a semiconductor material such as polycrystalline or amorphous silicon p-type or n weakly or strongly resistive, or a vanadium oxide (V ₂ O ₅ , VO ₂ ) developed in a semiconductor phase.

Finally, the support substrate 14 comprises an electronic circuit integrated on a silicon wafer, usually known as the "reading circuit". The reading circuit comprises on the one hand the stimulus and reading elements of the thermometric element 20, and on the other hand the multiplexing components which make it possible to serialize the signals coming from the different thermometric elements present in the matrix detector.

In operation, the membrane 12 heats up under the effect of incident electromagnetic radiation and the heating power produced is transmitted to the layer 20 of thermometric material. Periodically, the reading circuit arranged in the substrate 14 polarizes the membrane 12 by subjecting the nails 16 to a bias voltage and collects the current flowing in the thermometric element 20 to deduce a variation of its resistance and therefore the radiation incident to the origin of said variation.

The arrangement and operation of such a detector being conventional, it will not be explained in more detail for the sake of brevity. It should however be noted that the membrane 12 fulfills, in addition to the thermal insulation function, three main functions: that of antenna for the reception of the radiation, that of conversion of the electromagnetic power received into heating power and that of thermometric measurement of the heat output produced. Filling the role of antenna, the dimensions of the membrane 12 are therefore selected to be of the same order of magnitude as the wavelength of the radiation to be measured.

However, in the terahertz field, the wavelengths can reach the millimeter, which therefore requires a membrane of the same order of magnitude. However, for such dimensions, the heat mass, the mechanical strength and the radiation losses of the membrane are so problematic that they ultimately harm the efficiency of the detector.

Therefore, for such a range of frequencies, the radiation reception function is decoupled from other functions. The reception function is thus provided by a planar antenna and the function of converting the electromagnetic power into heating power is provided by the resistive load of the antenna. The dimensions of the load typically satisfy the impedance matching conditions which depend on the geometry of the antenna and the nature of the layers supporting it in order to obtain an optimal conversion. The resistive load is also in thermal contact with a thermometric element for measuring the heat output produced. The set then constitutes an antenna bolometer.

In such a configuration, the thermometric element is independent of the antenna and its size no longer depends on the incident wavelength but factors determining the intrinsic performance of the detector (sensitivity, signal-to-noise ratio, etc ... ), in line with the requirements of the intended application, for example active imaging or passive imaging.

Moreover, in most cases, the incident electromagnetic radiation is not polarized, so that its reception by a single antenna does not capture all of the electromagnetic power. However, a non-polarized radiation can be considered as the superposition of two components linearly polarized in two orthogonal directions, each of these components carrying half of the energy of the wave. As is known per se, an effective way of capturing incident electromagnetic radiation is to use two crossed butterfly antennas. The butterfly antenna is better known under the Anglo-Saxon " bow-tie ", and is explained for example on the thesis of R. PEREZ, available on the following website: http://www.unilim.fr/theses /2005/sciences/2005limo0053/perez_r.pdf.

The document US 6,329,655 describes an antenna bolometer 30, operating in the millimeter range and provided with two crossed butterfly antennas 32, 34, whose schematic views from above and in section are shown in FIGS. figures 2 and 3 respectively. The principle of the bolometer 30 is based on the capacitive coupling produced between the antennas 32, 34, arranged on a support substrate 36, and a resistive load 38, arranged in a suspended membrane 40 and on which a thermometric element 42 is disposed ( figure 3 ).

The resistive load 38, which takes the form of a square layer arranged in the center of the antennas 32, 34, has indeed a surface facing them and thus forms with the antennas a capacitance. The radiation picked up by the antennas 32, 34 is thus transmitted to the load 38 by capacitive coupling.

However, the shape of the resistive load poses problems of impedance matching.

où f est la fréquence du rayonnement, C est la valeur de la capacité formée entre les antennes 32, 34 et la charge résistive 38, et R est la valeur de la résistance de la charge résistive 38. It is estimated that the impedance matching, and therefore the capacitive coupling, is optimal for this load when the following relation is satisfied: $$\sqrt{{\left(\frac{\mathrm{1}}{\mathrm{\pi }\mathrm{.}\mathrm{f}\mathrm{.}\mathrm{VS}}\right)}^{\mathrm{2}}\mathrm{+}{\left(\mathrm{R}\mathrm{.}\mathrm{VS}\right)}^{\mathrm{2}}}\mathrm{\approx }\mathrm{100}\mathrm{\Omega }$$

where f is the frequency of the radiation, C is the value of the capacitance formed between the antennas 32, 34 and the resistive load 38, and R is the value of the resistance of the resistive load 38.

Increasing the value of the capacitor C to achieve impedance matching or optimum coupling is unsuitable since it assumes either a submicron space between the antennas 32, 34 and the load 38, a large overlap surface therebetween.

However, reducing the distance between the antennas and the load to a value between 100 and 200 nanometers presents difficulties linked to both physical phenomena (Casimir effect for mechanical stability, important radiative exchange leading to a degradation of the thermal insulation of the thermometric element and thus to a decrease of the sensitivity of the detector) than to current manufacturing techniques (control of the residual stresses of the layers to avoid unwanted contacts or control of sacrificial layers used to form the space between the antennas and the resistive load).

Moreover, increasing the size of the resistive load to increase the facing surfaces presents the same problems that led to the decoupling between the reception function and the conversion and thermometry functions previously mentioned. Therefore, for a terahertz adaptation, the value of the capacitance C is not free.

In such a configuration, the resistive load 38 must be weakly resistive, ie a square resistance of 50Ω to 200Ω, so as to ensure optimum coupling with terahertz radiation. This then results in construction of an undesired coupling, almost optimum, with the infrared radiation emitted by bodies at 300 ° K and very difficult to eliminate effectively without degrading the quality of the signal in the terahertz frequency range.

In fact, it is difficult to obtain, with a resistive load, associated with the two antennas and having a square shape in the center of the detector, optimum impedance matching and capacitive coupling, unless also optimal coupling of the detector with infrared radiation.

la permittivité ε sera très élevée, ce qui augmentera le couplage des deux antennes croisées. Ainsi, il n'est pas possible de réduire le découplage entre les antennes en choisissant le substrat en conséquence, sans par ailleurs modifier de manière importante l'agencement et le fonctionnement des éléments électroniques intégrés dans celui-ci.Moreover, the two antennas 32, 34 are both arranged on the substrate 36, they are coupled via it. It is thus observed that the gain of the detector is substantially reduced, leading to an unsatisfactory use thereof. For reasons of electrical connection, particularly ease of manufacture of the contact resumption between the read circuit arranged in the substrate and the thermometric element of the membrane, the substrate usually has a thickness as small as possible. However, since this thickness e must verify the relation e = λ / 4n with $\mathrm{not}=\sqrt{\mathrm{\epsilon }},$

the permittivity ε will be very high, which will increase the coupling of the two crossed antennas. Thus, it is not possible to reduce the decoupling between the antennas by choosing the substrate accordingly, without otherwise significantly alter the arrangement and operation of the electronic elements integrated therein.

The documents US 6329649 and Peytavit et al, The Joint 30th International Conference on Infrared and Milimeter Waves & 13th Intl. Conf. on Terahertz Electronics, Vol. 1, 19 Sep. 2005, pages 257-258 disclose bolometric antenna detectors.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a resistive bolometric detector with double crossed butterfly antennas not having any coupling between the antennas.

• ■ une première et une seconde antennes papillon croisées, destinées à collecter le rayonnement électromagnétique ;
• ■ une charge résistive couplée auxdites antennes pour convertir la puissance électromagnétique collectée en puissance calorifique ;
• ■ un micropont bolométrique suspendu au dessus d'un substrat par des bras de soutien et d'isolation thermique, le micropont comprenant :
  • o la charge résistive ;
  • o un élément bolométrique ou thermométrique couplé à la charge résistive pour s'échauffer sous l'effet de la puissance calorifique produite.

To this end, the subject of the invention is a bolometric detector of an electromagnetic radiation in the range extending from infrared to terahertz, comprising:

• ■ first and second crossed butterfly antennas for collecting electromagnetic radiation;
• A resistive load coupled to said antennas for converting the collected electromagnetic power into heating power;
• A bolometric microbridge suspended above a substrate by support and thermal insulation arms, the microbridge comprising:
  • o the resistive load;
  • a bolometric or thermometric element coupled to the resistive load to heat up under the effect of the heating power produced.

According to the invention, the first antenna is arranged outside the microbridge and in capacitive coupling with the resistive load, and the second antenna is arranged in the microbond in resistive coupling with the load.

By "microbridge" is meant here the structure suspended above the substrate and which therefore comprises in particular the bolometric membrane and the second antenna.

In other words, the antennas are separated from each other by the vacuum between the microbridge and the substrate so that there is a reduced coupling between them, in particular via a material.

According to a preferred embodiment of the invention, the resistive load comprises a metal film, and the microbridge comprises fins arranged opposite the first antenna on this metal film so as to achieve an impedance matching between the first antenna and the metallic film. Preferably, the fins take a shape similar to the central parts of the first antenna. In particular, the fins are covered with an electrical insulator and the bolometric element is at least partially arranged on said insulator and at least partially in contact with the metal film.

In other words, the fins are designed in size, shape and material to achieve optimum impedance matching with the first antenna, and this independently of the second antenna.

Thus, unlike the state of the art where the resistive load of an antenna performs both the impedance matching and the conversion of the electromagnetic power, and this for both antennas, it is provided according to the invention of additional elements dedicated to impedance matching of the first antenna. Having thus at least partially decoupled the impedance matching function of the conversion function, there is an additional degree of freedom to choose the resistive element plated against the fins also in charge of the conversion of the electromagnetic power, which is for example the metal film usually present in the microbridge for the electrical connection of the thermometric element on which the second antenna for example rests.

According to one embodiment of the invention, the resistive load comprises a metal layer and the second antenna is arranged at least partially on the metal film or metal layer, for example, that usually present for the electrical connection of the thermometric element with the reading circuit.

In other words, the resistive film here fulfills the conversion function for the electromagnetic power received by the second antenna, independently of the fins and the first antenna.

According to one embodiment of the invention, the first antenna is arranged on the substrate.

According to one embodiment of the invention, the first antenna is formed at least partially above the microbridge. The first antenna thus forms a screen for the thermometric element, which limits the absorption of parasitic radiation.

The subject of the invention is also a device for the matrix detection of an electromagnetic radiation in the range extending from the infrared to the terahertz, which, according to the invention, comprises a matrix with one or two dimensions of bolometric detectors of the type above.

BRIEF DESCRIPTION OF THE FIGURES

• ■ la figure 1 est une vue schématique en perspective d'un détecteur bolométrique élémentaire de l'état de la technique, déjà décrit ci-dessus;
• ■ les figures 2 et 3 sont des vues schématiques de dessus et en coupe d'un détecteur bolométrique à antennes selon l'état de la technique, déjà décrit ci-dessus ;
• ■ la figure 4 est une vue schématique en perspective d'un premier mode de réalisation d'un détecteur bolométrique selon l'invention ;
• ■ les figures 5 et 6 sont des vues en section du détecteur selon le premier mode de réalisation, respectivement selon les plans V-V et VI-VI de la figure 4 ;
• ■ la figure 7 est une vue schématique en perspective d'un deuxième mode de réalisation d'un détecteur bolométrique selon l'invention ;
• ■ la figure 8 est une vue schématique en section du détecteur selon le deuxième mode de réalisation selon le plan VIII-VIII de la figure 7 ;
• ■ la figure 9 est une vue schématique de dessus illustrant les ailettes d'adaptation d'impédance entrant dans la constitution du premier mode de réalisation ;
• ■ les figures 10 à 13 sont des vues schématiques en coupe illustrant un procédé de fabrication d'un détecteur selon l'invention.

The invention will be better understood on reading the following description, given solely by way of example, and made with reference to the accompanying drawings, in which identical references designate identical or similar elements, and in which:

• ■ the figure 1 is a schematic perspective view of an elementary bolometric detector of the state of the art, already described above;
• ■ the figures 2 and 3 are schematic views from above and in section of a bolometric antenna detector according to the state of the art, already described above;
• ■ the figure 4 is a schematic perspective view of a first embodiment of a bolometric detector according to the invention;
• ■ the Figures 5 and 6 are sectional views of the detector according to the first embodiment, respectively according to plans VV and VI-VI of the figure 4 ;
• ■ the figure 7 is a schematic perspective view of a second embodiment of a bolometric detector according to the invention;
• ■ the figure 8 is a diagrammatic sectional view of the detector according to the second embodiment according to the plan VIII-VIII of the figure 7 ;
• ■ the figure 9 is a schematic top view illustrating the impedance matching fins forming part of the constitution of the first embodiment;
• ■ the Figures 10 to 13 are schematic sectional views illustrating a method of manufacturing a detector according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

It will now be described, in relation to the Figures 4 to 7 , an elementary bolometric detector 50 according to a first embodiment of the invention, constituting a matrix of elementary detectors.

The bolometer 50 comprises an insulating substrate 52 on which is deposited a first plane butterfly antenna 54 made of conductive material, and a micropont 56, suspended above the substrate 52 by two conductive anchoring nails 58.

The microbridge 56 is formed of a central portion 60 and two heat-insulating arms 62 connecting the central portion 60 to the anchoring nails 58 and perpendicular to the main axis (VV) of the first antenna 54. The microbridge 56 comprises a first layer of electrical insulator 64, as well as a conductive layer 66, and more particularly a metal film, deposited on the insulating layer 64.

A second antenna 68 butterfly of conductive material, crossed with the first antenna 54 and main axis (VI-VI) parallel to the heat insulating arms 62, is also formed on the conductive layer 66 of the microbridge 56 and extends on either side of the central portion 60. The second throttle antenna 68 is thus in resistive coupling with the conductive layer 66.

The fins 70, 72, 74, made of the same material as the antennas 54, 68, are also provided on the conductive layer 66 with facing surfaces of the first butterfly antenna 54. The fins 70, 72, 74 are thus in position. capacitive coupling with the antenna 54 and are further selected to perform impedance matching therewith in a manner to be explained in more detail later.

The fins 70, 72, 74, as well as the portion of the second butterfly antenna 68 arranged in the central portion 60, are covered with an insulating layer 76 to electrically isolate them, a portion of the conductive layer 66 being left free .

A layer of thermometric material 78 is also deposited on the insulating layer 76 in contact with the conductive layer 66 at the portion of the latter left free by the insulating layer 76.

Finally, the substrate 52 comprises an insulating layer 80, having a low absorption coefficient in the range of operating wavelengths of the detector, and a reflector 82, the layer 80 and the reflector 82 forming a resonant cavity for the antennas 54, 68 in the frequency range of interest. A functional layer 84 comprising the detector reading circuits is finally provided under the reflector 82.

The figures 7 and 8 are schematic views in perspective and in section of a second embodiment according to the invention. This second embodiment differs from the first embodiment by the location of the first antenna 54. This is arranged above the microbond 56 by means of a support structure 92, rather than disposed on the substrate 52. allows in particular to form a screen for the thermometric element 78, limiting the absorption of parasitic radiation.

More particularly, the first antenna 54 forms a screen above the heat-insulating arms 62, anchoring nails 58 and contacts between the thermometric element 78 and the conductive layer 66, which is particularly advantageous in measuring where these elements usually have resistance characteristics sensitive to infrared radiation located in the 8μm-14μm frequency band.

It will be noted that the resistive load of the antennas 54, 68 is defined by this conducting layer 66, and in particular by the exposed zones of this layer.

In operation, electromagnetic radiation is picked up by the butterfly antennas 54, 68. The electromagnetic power picked up by the first antenna 54 is then transmitted to the fins 70, 72, 74 by capacitive coupling. The electromagnetic power transmitted to the fins 70, 72, 74 is then converted into heat by the conductive layer 66 on which the fins are formed.

The electromagnetic power picked up by the second antenna 68 is itself directly converted by this conductive layer 66 on which it rests. The thermometric element 78, which is in contact with the conductive layer 66, then undergoes because of said contact, a heating and thus sees its modified resistance. The conducting layer 66, which also fulfills the function of polarization electrode of the thermometric element 78, is then regularly biased to subject the thermometric element 78 to a bias voltage and thereby circulate a current therein. to know its variation of resistance, as is known per se.

The figure 9 schematically illustrates in a view from above the first and second butterfly antennas 54, 68, as well as the fins 70, 72, 74. As can be seen, a first central wing 72, of rectangular shape, straddles the two wings 100, 102 of the butterfly antenna 54, and two lateral wings 70, 74 are respectively facing portions 100, 102 of the first antenna 54. Preferably, the lateral wings 70, 74 are substantially of shape and size identical to a part of the antenna 54. The fins have a trapezoidal shape equivalent to the trapezoidal part of the antenna located opposite. Its surface advantageously corresponds to the capacitance C necessary for impedance matching. Thus, an optimal impedance match is obtained.

In addition, the resistive load of the antennas, defined by the portions of the conductive layer 66 located between the fins 70, 72, 74 and between the parts of the second antenna 68, is of reduced surface area. This area being reduced, the coupling of the detector according to the invention with the infrared radiation, which is in first approximation proportional to the size of the resistive load, is also reduced.

Furthermore, the length of the antennas 54, 68 and fins, as well as their opening angle θ are chosen so as to increase or reduce the bandwidth of the detector.

It will now be described in relation to the Figures 10 to 12 a method of manufacturing the detector according to the first embodiment according to the invention.

As illustrated in figure 10 , the cavity 80, 82 of the detector is formed of a reflector 82 disposed on the reading circuit 84, such as for example an aluminum layer, and a layer 80 of insulating material, having a coefficient of absorption of lowest possible in the range of operating wavelengths of the detector. For example, layer 80 consists of SiO, SiO ₂ , SiN, Ta ₂ O ₅ , Ta ₂ O ₅ -TiO ₂ , HfO ₂ , SrTiO ₃ , Ba _1-x Sr _x TiO ₃ or a mixture of these.

où :

• ■ λ est une longueur d'onde de la gamme de fonctionnement du détecteur, par exemple la longueur d'onde centrale de cette gamme ; et
• ■ $\mathrm{n}=\sqrt{\mathrm{\epsilon }},$
  
  ε étant la permittivité diélectrique du matériau constitutif de la couche 80.

The layer 80 also has a thickness of 1 .mu.m to 500 .mu.m, set at the value: $$\mathrm{\lambda }/\left(4\mathrm{not}\right)$$

or :

• ■ λ is a wavelength of the operating range of the detector, for example the central wavelength of this range; and
• ■ $\mathrm{not}=\sqrt{\mathrm{\epsilon }},$
  
  ε being the dielectric permittivity of the material constituting the layer 80.

A resonant cavity is thus obtained for the terahertz radiation object of the detection.

The layer 80 is also traversed by electrical connections 110, for example in the extension of the anchoring nails 58, so as to electrically connect the read circuit 84 and the thermometric element 78. For example, vias are made in layer 80 according to a conventional technique, and the vias thus produced are filled with a metal such as tungsten, aluminum or copper using a damascene technology associated with a planarization technique.

The first antenna 54, made of conducting material such as aluminum, tungsten silicide, titanium or the like, is formed on the layer 80 by a conventional photolithography technique and has a thickness of between 0.1 .mu.m and 0. 5 .mu.m.

Once the substrate 52 and the first antenna 54 have been manufactured, a sacrificial layer 112 ( figure 11 ), for example made of polyimide, with a thickness of between 0.5 μm and 5 μm, is formed on the antenna 54 and the layer 80. The thickness of the sacrificial layer is chosen to achieve a high performance capacitive coupling between the first antenna 54 deposited on the substrate 52 and the fins 70, 72, 74 which will be subsequently formed. This thickness is chosen as low as possible while being compatible with the mechanical strength of the microbridge (electrostatic bonding).

An insulating layer 64 is then deposited on the sacrificial layer 112, then a thin metal film 66, for example made of Ti, TiN, Pt, NiCr or others, is deposited on the insulating layer 64.

As described above, the metal film 66 performs the function of supplying electricity and reading the thermometric element 78 via the heat-insulating arms 62 and the resistive charging function from its surface in contact with the fins 70, 72 , 74 and the second throttle antenna 68. The insulating layer 64 and the thin film 66, each of a thickness between 0.005 .mu.m and 0.05 .mu.m, are preferably deposited by PECVD (for the English expression " Plasma Enhanced Chemical Vapor Deposition ") or cathodic sputtering, then etched chemically or plasma to form the insulating arms 62. The metal film 66 is also etched chemically or plasma to form a central portion 114, on which will be formed the fins 70 , 72, 74 and the second antenna 68, and side portions 116, which will be in contact with the thermometric element 78 for its power supply and its reading.

The square resistance of the metal film 66 is advantageously chosen so as to provide effective thermal insulation of the microbridge 56 with respect to the reading circuit 84. Preferably, the square resistance of the metal film 66 is between 100 Ω / square and 500 Ω / square, because it is the value of the access resistors of the materials used to improve the thermal resistances (bolometer arm). Finally, the metal film 66 is connected to the reading circuit 84 by means of the conductive anchoring nails 58, produced through the sacrificial layer 112 in a similar manner to the connections 110, and electrical connections 110.

The second antenna 68 and the fins 70, 72, 74 are made of a conductive material such as aluminum, tungsten silicide, titanium or others. For their formation, a layer of the conductive material, with a thickness of between 0.1 .mu.m and 0.5 .mu.m, is deposited on the central portion 114 of the metal film 66 by cathodic sputtering or by thermal decomposition (LPCVD for the English expression. Saxon " Low Pressure Chemical Vapor Deposition ") or plasma decomposition (PECVD), then the second antenna and the fins are formed by chemical etching, plasma etching or by a technique of "lift off" type of said layer. In a variant, the antenna and the fins consist of metal multilayers. The antenna 68 and fins 70, 72, 74 thus formed define regions that convert the electromagnetic radiation into electrical current and define exposed areas of the metal film 66 that convert the electric current into thermal energy.

The second antenna 68 and the fins 70, 72, 74 are then covered with a layer of insulating material 76, such as SiN, SiO, ZnS or others. The layer 76, having a thickness of between 0.005 μm and 0.1 μm, is made in order to avoid any short circuit between the fins 70, 72, 74 and the thermometric element 78. The layer 76 is for example made to using a low temperature deposition technique such as sputtering or plasma decomposition (PECVD). The layer 76 is then etched, for example chemically or by plasma, to reveal the lateral portions 116 of the metal film 66 to which the thermometric element 78 will be connected , as well as the heat-insulating arms 62.

The thermometric element 78 is then deposited on the layer 76 and the side portions 116 using, for example, a low temperature deposition technique such as spraying. The constituent material of the thermometric element 78 is, for example, an amorphous or polycrystalline semiconductor, such as Si, Ge, SiC, a-Si: H, a-SiGe: H, a metallic material or a vanadium oxide or a magnetite oxide. This material must have a non-zero temperature coefficient resistance (TCR). In other words, it has a resistance that varies with temperature.

Finally, the sacrificial layer 112 is removed, the nature of which determines the release technique, and preferably by chemical etching or plasma.

As can be seen, the etching of the constituent materials of the detector according to the invention is carried out predominantly or exclusively, by etching techniques, possibly assisted by plasma, these techniques for obtaining accurate and reproducible etchings.

When the first butterfly antenna 54 is located above the microbond 56, a sacrificial layer 130 ( figure 13 ) is deposited on the entire microbridge 56 and the first sacrificial layer 112 used to develop it. The sacrificial layers 112, 130 are then etched to produce the supports 92 of the antenna 54. The supports 92 may for example be made of a material different from that of the antenna 54, for example an insulating material deposited by thermal decomposition (LPCVD). Finally, the butterfly antenna 54 is formed by deposition and etching of a conductive layer, as described above, and the sacrificial layers removed.

• ■ un découplage des antennes papillons qui ne sont plus sur le même plan et ne sont plus déposées sur un même support ;
• ■ une adaptation d'impédance optimale grâce aux ailettes, l'adaptation d'impédance étant en outre réalisée de manière indépendante pour chacune des antennes ;
• ■ une surface de la charge résistive très faible, réduisant ainsi le couplage du détecteur avec le rayonnement infrarouge qui est en première approximation proportionnel à la surface de charge.

Thanks to the invention, it is thus obtained:

• Decoupling of the butterfly antennas which are no longer on the same plane and are no longer deposited on the same support;
• ■ Optimum impedance matching thanks to the fins, the impedance matching being furthermore performed independently for each of the antennas;
• ■ a very low resistive load surface, thus reducing the coupling of the detector with the infrared radiation which is in first approximation proportional to the load surface.

Claims (7)

1. A bolometric detector for detecting electromagnetic radiation in the region extending from infrared to terahertz frequencies, comprising:

   ■ a first and a second crossed bow-tie antenna (54, 68) intended to collect electromagnetic radiation;

   ■ a resistive load (66) coupled to said antennas (54, 68) in order to convert the collected electromagnetic power into calorific power;

   ■ a bolometric micro bridge structure (56) suspended above substrate (52) by support and thermal isolation arms (62) with the micro bridge comprising:

   o the resistive load (66);

   o a thermometric element (78) coupled to resistive load (66) so that its temperature can rise due to the effect of the calorific power produced ;

   characterized in that first antenna (54) is located outside the micro bridge (56) and is capacitively coupled with resistive load (66) and in that second antenna (68) is located in micro bridge (56) and is resistively coupled with the resistive load (66).
2. The bolometric detector as claimed in claim 1, characterized in that the resistive load comprises metal film (66) and in that micro bridge (56) comprises winglets (70, 72, 74) arranged facing first antenna (54) on metal film (66) so as to obtain impedance matching between first antenna (54) and metal film (66).
3. The bolometric detector as claimed in claim 2, characterized in that winglets (70, 72, 74) are covered in an electrically insulating material (76), thermometric element (78) being placed at least partially on said insulator (76) and being at least partially in contact with metal film (66).
4. The bolometric detector as claimed in claim 1, 2 or 3, characterized in that the resistive load comprises metal film (66) and in that second antenna (68) is placed at least partially on this metal film (66).
5. The bolometric detector as claimed in any of the above claims, characterized in that first antenna (54) is placed on substrate (52).
6. The bolometric detector as claimed in any of claims 1 to 3, characterized in that first antenna (54) is formed at least partially above the micro bridge (56).
7. An array detector device for detecting electromagnetic radiation in the region extending from infrared to terahertz frequencies, characterized in that it comprises a one or two dimensional array of bolometric detectors in accordance with any of the above claims.

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